1. Field of the Invention
The present invention relates to a lamp unit and, more particularly, to an improvement of a cooling mechanism and a lamp replacing mechanism for a short arc type lamp.
2. Description of the Related Art
As a light source of an optical analyzer such as a fluorescence detector, short arc type lamps such as a xenon lamp, metal halide lamp and a mercury lamp are generally used.
FIG. 5 shows a general structure of a xenon short arc type lamp (hereinunder referred to as "xenon lamp"). As shown in FIG. 5, a xenon lamp 10 is composed of a long and narrow bulb 12, electrode bases 14a, 14b into which both ends of the bulb 12 are fitted, and electrode cores 16a, 16b implanted in the electrode bases 14a and 14b, respectively.
A spherical bulb portion 13 filled with xenon gas is formed at the center of the bulb 12, and the tips of the electrode cores 16a and 16b are in close proximity with each other in the spherical bulb portion 13.
A high voltage is applied to the electrode bases 14a, 14b so as to cause arc discharge between the electrode cores 16a and 16b and to light the xenon lamp 10 thereby.
When the xenon lamp 10 is lighted, a large quantity of heat is produced between the electrode cores 16a, 16b where arc discharge is caused, so that the temperature of the tube wall of the spherical bulb portion 13 rises. For example, in the case of a 150-W xenon lamp, the pressure in the spherical bulb portion 13 reaches as high as 40 to 50 atm. Therefore, if the heat dissipation at the tube wall of the spherical bulb portion 13 is insufficient, there is a fear of the spherical bulb portion 13 bursting due to a temperature rise.
If the temperature in the spherical bulb portion 13 abnormally rises, there is a fear that the property of the glass constituting the spherical bulb portion 13 changes, which may lead to what is called devitrification.
The rise in the temperature of the tube wall of the spherical bulb portion 13 is different between in the lamp (hereinunder referred to as "vertically installed lamp") which is installed in a lamphouse 18 in such a manner that the longitudinal portion of the bulb 12 is perpendicular to the bottom of the lamphouse 18, as shown in FIG. 6 and the lamp (hereinunder referred to as "horizontally installed lamp") which is installed in the lamphouse 18 in such a manner that the longitudinal portion of the bulb 12 is horizontal to the bottom of the lamphouse 18, as shown in FIG. 7.
When the xenon lamp 10 is lighted, the convection current of the xenon gas indicated by the arrows in FIGS. 6 and 7 is produced in the spherical bulb portion 13 due to the heat produced from the arc 20, and the heat of the arc 20 is carried on the convection current upward in the spherical bulb portion 13.
In the vertically installed lamp shown in FIG. 6, since the heat carried upward in the spherical bulb portion 13 is diffused in a comparatively wide area (hatched portions 13a in FIG. 6) in the vicinity of the neck portion of the spherical bulb portion 13 and dispersed, it is possible to comparatively suppress the rise in the temperature in the spherical bulb portion 13.
On the other hand, in the horizontally installed lamp shown in FIG. 7, since the heat carried upward in the spherical bulb portion 13 concentrates on a certain limited point (hatched portions 13b in FIG. 7) of the spherical bulb portion 13, the heat dissipation efficiency is low, which results in an abnormal temperature rise in the spherical bulb portion 13.
For this reason, in a conventional optical analyzer, the xenon lamp 10 is vertically installed in the lamphouse 18 so as to prevent an extreme temperature rise in the spherical bulb portion 13.
The vertically installed lamp 10 in an optical analyzer, however, suffers from various problems.
Firstly, if the xenon lamp 10 is vertically installed in the lamphouse 18, the height of the lamphouse 18 becomes larger than the length of the bulb 12, so that the height of the optical analyzer itself becomes large. In other words, it is inevitable to increase the size of the optical analyzer.
Secondly, it is necessary to limit the height of the optical analyzer to the height of the lamphouse 18 in order to hold down the increase in the size of the optical analyzer to the minimum. It is therefore impossible to dispose an optical system, an electric system, etc. in the upper portion or the lower portion of the lamphouse, so that the degree of freedom in designing is restricted.
Thirdly, since it is often the case that another apparatus is laid on the optical analyzer for analysis and measurement, it is desirable to provide a lamp insertion hole not on the upper surface of the analyzer but on the side surface thereof so as to facilitate the replacement of the xenon lamp 10. It is further desirable from the point of view of the degree of freedom in designing and the mechanical strength that the lamp insertion hole is as small as possible so long as it is sufficiently large to facilitate the replacement of the xenon lamp 10. However, if the xenon lamp 10 is vertically installed in the lamphouse 18, when the lamp insertion hole is provided on the side surface of the analyzer, the diameter of the lamp insertion hole becomes larger than the length of the xenon lamp 10, so that the degree of freedom in designing and the mechanical strength are greatly lowered. Therefore, in order to reduce the diameter of the lamp insertion hole, it is necessary to provide the lamp insertion hole on the upper surface of the analyzer.
In this way, the vertically installed xenon lamp 10 has various problems.
These problems are solved by horizontally installing the xenon lamp 10 in the lamphouse 18, but in this case, the problem of an extreme temperature rise in the spherical bulb portion 13 remains unsolved.
If a pulsation lighting method is adopted to the xenon lamp 10, the calorific power of the arc is reduced and the problem of a temperature rise is solved, but this method is not practical because the intensity of light at each flash is not uniform, thereby producing large noise.
In addition, the horizontally installed xenon lamp 10 produces a problem that arc produced between the electrode cores 16a, 16b distorts as well as the problem of a temperature rise. This is because the arc 20 is largely bent upward due to the convection current of the xenon gas, as shown in FIG. 7, while in the vertically installed xenon lamp 10, the arc 20 is produced symmetrically with respect to the center axis of the xenon lamp 10, as shown in FIG. 6.
As a result, the light emission of the horizontally installed xenon lamp 10 becomes unstable, which is a cause of an increase in the noise of the analyzer, and it is impossible to efficiently collect the light emitted from the arc 20 due to the distortion of the arc 20.
If the curve of the arc 20 becomes extremely large, the arc 20 comes into contact with the tube wall of the spherical bulb portion 13, so that the temperature of the tube wall further rises, thereby enhancing the risk of a burst of the spherical bulb portion 13 and devitrification of the glass.
Even in the case of horizontally installing the xenon lamp 10 in the lamphouse 18, although it is possible to make the diameter of the lamp insertion hole comparatively small, a problem still remains unsolved that the lamp insertion hole is formed at a certain cost of the degree of freedom in designing and the mechanical strength.